A semiconductor integrated circuit chip is typically fabricated with a back-end-of-line (BEOL) interconnect structure, which comprises multiple levels of metal lines and inter-level metal vias, to connect various integrated circuit components and devices that are fabricated as part of a front-end-of-line (FEOL) layer of the semiconductor integrated circuit chip. Current state of the art BEOL process technologies typically implement copper to form BEOL interconnects, as the use of copper material is known to significantly reduce resistance in the BEOL interconnect structure, resulting in improved conduction and higher performance. As copper interconnect structures are scaled down, however, there is a significant increase in the resistivity and current density within the copper interconnect structures, which is undesirable. The increase in current density in copper interconnect structures causes an increase in the current-driven electromigration of copper atoms. In the context of copper interconnect structures, electromigration is the transport of copper atoms caused by the gradual movement of ions in the copper material due to momentum transfer between conducting electrons and diffusing copper atoms. The electromigration of copper atoms can lead to various defects in copper interconnect structures such as voids and hillock defects.
Based on Matthiessen's Rule (an empirical rule), the total resistivity ρt of a given metallic material is the sum of a bulk resistivity ρb of the metallic material, a resistivity ρs of the metallic material due to surface scattering of conduction electrons, and a resistivity ρg of the metallic material due to scattering of conduction electrons at grain boundaries, i.e., ρt=ρb+ρs+ρg. In this regard, the electrical resistivity of a metallic interconnect structure depends, in part, on a grain microstructure of the metallic material which forms the metallic interconnect structure. For example, a polycrystalline microstructure is one which comprises many crystallites (or grains) of varying size and orientation, and with random texture and no grain direction. A polycrystalline microstructure tends to decrease the electrical conductivity of the metallic material, as well as increase electromigration within the metallic material due to the electron diffusion paths that exist along the various grain boundaries in the polycrystalline metallic material. In addition, it is known that the electrical resistivity of a narrow metal wire is significantly affected by electron surface scattering if the cross-sectional dimension of the metal wire is smaller than the mean free path for electron-phonon collisions. The resulting increase in resistivity in metallic interconnect structures with smaller dimensions is a major challenge for nano-scaling of BEOL interconnect structures.